When Charles Darwin arrived at the Galápagos Islands in 1839, he had the opportunity to observe the habits of frigatebirds, and marvelled at their graceful flight manoeuvres and their ability to soar up high. “When it sees any object on the surface of the water,” he wrote, “[it] descends from a great height… with the swiftness of an arrow; and at the instant of seizing with its long beak and outstretched neck, the floating morsel, it turns upwards, with extraordinary dexterity, by the aid of its forked tail, and its long, powerful wings.”

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He also noticed that they can fly for weeks without ever coming to rest. “It never touches the water with its wings, or even with its feet; indeed I never heard of one having been seen on the surface of the sea.” Many other bird species can also take long-haul flights lasting days, weeks, or even longer, and it is widely assumed that they sleep on the wing.

A new study by researchers in Germany now provides the first direct evidence that frigatebirds do indeed sleep during periods of prolonged flight.

Niels Rattenborg of the Max Planck Institute for Ornithology in Seeweisen and his colleagues travelled to the Galápagos Islands and caught 15 adult female great frigatebirds from their nests. They anaesthetised each of the birds and fitted each with a head-mounted data logger containing miniaturised electroencephalogram sensors and an accelerometer, then released them again. This allowed them to record the birds’ brainwaves and movements while they flew over the Pacific Ocean.

Frigatebirds have stereotypical flight patterns, circling on thermal air columns to soar up into the sky, and then gliding back down in straight lines. This is exactly what the researchers observed. All 15 birds took one or two trips over waters to the north east of the islands, taking roughly clockwise trajectories.

These trips lasted up to 10 days, during which time the birds covered distances of up to 3,000km. Their altitudes did not differ markedly between day and night, but the researchers noted that they tended to circle more in daylight hours, and soar the highest during the hour before sunset.

For every 24-hour period, the birds spent an average of about 40 minutes sleeping. They slept exclusively at night, and typically when they were ascending. The sleep episodes that occurred during circling flight lasted an average of about 12 seconds, while those that occurred during straight flight lasted about 7 seconds.

This is less than one tenth of the time they spend sleeping on land. In-flight sleep was also far less intense than sleep on land; the birds sacrifice sleep for vigilance, and incur a large sleep ‘debt’ during long distance flights, which they may compensate for once back on land.

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Other animals have evolved similar strategies. For example, male polygynous pectoral sandpipers dramatically reduce the time they spend sleeping during their three-week mating season, giving them more time to compete for females. As a result, the males that spend the least time sleeping are the ones that sire the most offspring. It’s also well established that dolphins sleep with one brain hemisphere at a time, and recent research suggests that we may also do the same when sleeping in a strange place, in order to be more vigilant.

Despite being marine birds, frigatebirds are not particularly well adapted to the aquatic life - they have poorly-webbed feet, and their feathers aren’t waterproof. Their brief periods of in-flight sleep may serve as power naps that counter the effects of sleep deprivation during long distance flight, and understanding how they manage on such little sleep could teach us more about the effects of sleep deprivation in humans.